Brushless motors such as switched reluctance motors are increasingly demanded recently because they are inexpensive and simple in structure. Those motors incorporate an encoder for outputting a pulse signal in synchronism with the rotation of the rotor, which is rotated by sequentially switching the current supply phase by counting pulses of a pulse signal of the encoder and detecting the rotation position of the rotor on the basis of the count. Enabling detection of the rotation position of the rotor on the basis of the encoder count after starting, motors of this type having an encoder are used as drive sources of various position switching devices in which a position switching control (positioning control) for rotating the rotor to a target position by means of a feedback control system (i.e., a closed-loop control system) is performed.
During a feedback control for rotating the rotor to a target position on the basis of the encoder count, if, for example, a pulse signal of the encoder is lost temporarily for a certain reason or a noise pulse occurring on a signal line of the encoder is erroneously recognized as a regular pulse, the synchronization between the current supply phase (encoder count) and the rotation phase of the rotor is lost and the rotor can no longer be driven normally. As a result, the rotor may stop or rotate in reverse to cause a state that the feedback control cannot be continued normally. In such a case, the motor becomes uncontrollable.
Further, if a disconnection occurs in the drive coil of a motor or their drive circuit fails, the motor cannot be driven normally. In view of this, JP-A-2001-271917 proposes a fail-safe circuit in which the stator core of a motor is equipped with two independent systems of drive coils and two systems of drive circuits for driving the respective systems of drive coils separately are provided. When a failure occurs in the drive coil or drive circuit of one system, the rotation position of the rotor is feedback-controlled by using the drive coil of the other system and the drive circuit of the other system that are free from failure.
However, when a failure occurs in the drive coil or drive circuit of one system and the rotation position of the rotor is feedback-controlled by using only the drive coil of the other system, the driving torque becomes a half of that in a normal state. Because of insufficient driving torque, the rotation position of the rotor may not be feedback-controlled. However, to secure sufficient driving torque that enables a normal feedback control only with the drive coil of a single system, a large drive coil is needed and hence the motor becomes unduly large.